As automotive companies and others strive to improve engine efficiency and decrease engine emissions, interest has focused on rotary engines, i.e., internal combustion engines wherein a rotor (rotary piston) rotates within a housing, with one or more combustion chambers being formed between the rotor and housing to travel about the housing as the rotor rotates. Perhaps the best-known type of rotary engine is the Wankel engine, wherein a somewhat triangular rotor rotates eccentrically (i.e., such that its axis of rotation does not coincide with its geometric axis) within a housing having a somewhat oval-shaped interior. (Though other types of rotor and housing configurations are also possible, e.g., a generally square rotor within a housing having a “cloverleaf” interior; see, e.g., U.S. Pat. No. 2,988,065 to Wankel et al.) Rotary engines are of interest because they are relatively compact and light-weight compared to reciprocating-piston engines having similar output, making rotary engines an attractive possibility for use in hybrid vehicles (vehicles which use internal combustion engines in combination with other energy sources, typically electric batteries, to provide their motive power). In particular, rotary engines would seem to be promising for use in battery-powered electric vehicles to extend their range when their batteries begin running low. However, rotary engines have fuel efficiency and pollutant emissions drawbacks which have prevented their widespread adoption: the high heat loss from the relatively large surface area of the combustion chamber, and pressure losses from poor sealing between engine chambers, serve to hinder engine output; and problems arising from the elongated shape of the combustion chamber, such as flame quenching (i.e., poor combustion propagation) and extended combustion duration, tend to cause high soot emissions (emissions of unburned or partially burned hydrocarbons), as well as serving as further efficiency hindrances.
Interest in achieving greater engine efficiency has also led to efforts to improve diesel (compression ignition) engines. (For the reader having limited familiarity with internal combustion engines, the primary difference between gasoline engines and diesel engines is the manner in which combustion is initiated. Gasoline engines, also commonly referred to as spark ignition or “SI” engines, provide a relatively fuel-rich mixture of air and fuel into an engine cylinder, with a spark then igniting the mixture to drive the piston outwardly from the cylinder to generate work. In diesel engines, also known as compression ignition engines, fuel is introduced into an engine cylinder as the piston compresses the air therein, with the fuel then igniting under the compressed high pressure/high temperature conditions to drive the piston outwardly from the cylinder to generate work.) Diesel engines tend to be more efficient than gasoline engines, providing admirably high power output per fuel consumption, but they unfortunately tend to have high pollutant emissions, in particular emissions of soot and nitrogen oxides (commonly denoted NOx). Soot is generally associated with incomplete combustion, and can therefore be reduced by increasing combustion temperatures, or by providing more oxygen to promote oxidation of the soot particles. NOx, which tends to cause adverse effects such as acid rain, is generally associated with high-temperature engine conditions, and may therefore be reduced by use of measures such as exhaust gas recirculation (EGR), wherein the engine intake air is diluted with relatively inert exhaust gas (generally after cooling the exhaust gas), thereby reducing the oxygen in the combustion chamber and reducing the maximum combustion temperature. Unfortunately, measures which reduce soot production in an engine tend to increase NOx production, and measures which reduce NOx production in an engine tend to increase soot production, resulting in what is often termed the “soot-NOx tradeoff.” NOx and soot can also be addressed after they leave the engine (e.g., in the exhaust stream), but such “after-treatment” methods tend to be expensive to install and maintain. As examples, the exhaust stream may be treated with catalysts and/or injections of urea or other reducing/reacting agents to reduce NOx emissions, and/or fuel can periodically be injected and ignited in the exhaust stream to burn off soot collected in “particulate traps” (which tend to hinder fuel efficiency). Because these approaches require considerable complexity, hybrid vehicles using diesel engines as range extenders tend to be expensive.